How to measure static friction and sliding and rolling friction? Calculation formula So under what circumstances is the force acting on an object equal to static friction?

How to measure static friction and sliding and rolling friction? Calculation formula So under what circumstances is the force acting on an object equal to static friction?

Static friction is obtained by measuring other external forces through force balance
Sliding friction is generally measured by uniform linear motion, which is done in the laboratory. Sliding friction coefficient can be measured by this method
Generally, the rolling friction is not measured and is very small

How to distinguish static friction, sliding friction and rolling friction?
For example, I always confuse rolling friction with sliding friction, and rolling friction with static friction. I always think that rolling friction can not be said to exist. I can only say that there is rolling friction. I think sometimes static friction shows rolling friction (for example, a cylinder rolls on a rough horizontal plane without sliding), Sometimes sliding friction takes the form of rolling friction (for example, a cylinder rolling on a rough horizontal surface)!

Static friction does not necessarily cause relative displacement. Sliding friction and rolling friction cause relative displacement
Sliding friction means that the contact surface between two objects with relative displacement remains unchanged;
Rolling friction changes the contact surface
For example, when a cylinder rolls on a rough horizontal surface, if there is no sliding rolling, there will be relative displacement, change of contact surface, and rolling friction; if there is sliding rolling, there will be relative displacement, and some contact surfaces will not change, and there will be rolling friction + sliding friction
get it?

The product of x ^ 2 + MX + N and polynomial x ^ 2-3x + 2 does not contain x ^ 2. The value of M and N can be obtained by x ^ 3 term

After multiplication of two polynomials, add all x ^ 2 terms to get (2-3m + n) x ^ 2, and also add all x ^ 3 terms to get (- 3 + m) x ^ 3
The problem does not contain x ^ 2, x ^ 3, so there are 2-3m + n = 0, - 3 + M = 0 joint solution, M = 3, n = 7
(end)

The total mass of a bottle is 140g after it can be filled with water. The mass and volume of the bottle can be calculated after it is filled with 120g alcohol

Let v be the volume
Water density ρ 1 = 1g / cm ^ 2, alcohol density ρ 2 = 0.8g / cm ^ 2
(ρ1-ρ2)V=140-120
V=100cm^2
m=140-100=40g

4 out of 9 and 1 out of 3

Four out of nine and three out of nine

Three minus eight of eleven times three minus eight of eleven

3-(8/11)*(3/8)-8/11
=3-((8/11)*(3/8)+8/11)
=3-(3/11+8/11)
=3-1
=2

An applied problem about quadratic of one variable
For a rectangular sheet of iron with a length of 100 cm and a width of 50 cm, cut off four equal small squares at its four corners and fold it into a cuboid tank without a cover, so that its bottom area is 3600 square meters. Find the side length of the cut-off square. Let the side length of the cut-off square be xcm

First draw a picture and mark the number and X
(100-2x)(50-2x)=3600
4x²-300x+1400=0
2x²-150x+700=0
(2x-10)(x-70)=0
Ψ x = 5 or x = 70 (round)

What is the least common multiple of 22, 40 and 56?
There are also common multiples of 8, 12 and 30

3080
one hundred and twenty

25+75-25+75=（25-25）+（75+75）=0+150=150______ ．

25 + 75-25 + 75, = (25-25) + (75 + 75), = 0 + 150, = 150, so the answer is: √

-35 * 74 of 99 and 37 (calculated by simple method)

=-(100-2/37)*74
=-(100*74-2/37*74)
=-(7400-4)
=-7306